Add support for a cache version parameter in JS/CSS URLs

This PR adds the possibility to add a kind of "uuid" GET parameter (actually named "version") to the JS or CSS (or whatever) files loaded by the application, thus enabling to force the browser to load new files after running the buildout command.
Actually this line would need to be fixed since the "extra_params" is something like "?lang=de" we may get the following incorrect URL:
http://foobar/?version=5c783d81aaa945b2adc751e10cb51174?lang=de (additional "?" instead of "&").

I wonder if we could not change the way "extra_params" is sent to the template: use a dict instead of a string:
https://github.com/camptocamp/c2cgeoportal/blob/master/c2cgeoportal/views/entry.py#L730
and pass it to "request.route_url" using the _query parameter.